10 HP Outboard Motor Powering A 100 lbs Boat?

This setup works perfectly, but to get it to this stage isn’t easy.

At 100 lbs, the S4 Microskiff is the world’s most lightweight high-performance microskiff, and its official power rating is up to 6 HP. So why try to overpower it? The answer to this is related neither to hydrodynamics nor to engineering, but to human psychology: Some of us like speed. We enjoy traveling fast, and the faster the better.
And while the S4 running a 6 HP is pretty fast, it’s faster with a 9.8 HP motor, providing you succeed in getting it ready for running such a bigger, heavier, and much more powerful motor.
Such project would include beefing up the boat’s transom, as well as blocking and plugging every way that spray and water can shoot up from the propeller shaft area and into the back of the cockpit, which isn’t a trivial project.
This post’s featured image (see above) shows parts of the structures added at the back of the transom, between the hull tips, and around the motor’s shaft itself.
Read more: S4 Microskiff Powered By A 9.8 HP Tohatsu Outboard Motor

Bear in mind that any boat that’s powered with a motor, or motors that exceed its official power rating is considered overpowered, and in some states overpowering a boat is illegal. Needless to say that anyone who wishes to overpower their boat should first check the legal status of such a project in their state.

Also, not all outboard motors in the 10 HP class are suitable for use with the S4. The two things you need to check are the weight of the motor, and the size of its propeller shaft. After this examination, two motors make it to the finish line: the 9.8 HP Tohatsu, and the 9.8 HP Mercury, each weighing 85 lbs. Remember that the S4 works only with 20″ Long (L) shaft motors.

Typically, small craft powered by 10 HP motors are limited to speeds of up to a little over 20 mph, at best, and this can be achieved only on perfectly flat water, and with a lightweight driver. In the case of the ultralight S4, the driver is the heaviest part of the trio comprising boat, motor and driver, and due to hydrodynamics that favor lightweight vessels, the driver’s weight has a most noticeable effect on the boat’s speed, for better and for worse.
Outfitting the motor with a propeller of the biggest diameter and pitch will certainly help, but considering the motor’s max RPM and propeller’s slippage, the chances of going faster than 20 mph are very slim, and for heavy drivers they are practically zero.

Pros and Cons of a big motor –
  • Cons – Potential legal issues in some states, technical issues including non-trivial ones related to safety, a bigger motor is much heavier than the recommended 6 HP motor, and for many people this could be a problem in terms of carrying it.
  • Pros – Driving an S4 microskiff powered by a bigger motor is more fun, at least for someone who likes speed and is capable of safely driving such craft.


Transom Design For Powerful Outboard Motors

This article discusses factors that should be taken into account in the design and reinforcement of a boat’s transom, and by boat we mean any motorized small vessel, including microskiffs, motorized kayaks, and canoes.

The Outboard Motor Effect on a Boat’s Transom

An outboard motor is attached to the top part of the transom (or vertical mounting plate) by means of two screws. The lower part of the motor’s mounting bracket is pressed against the back side on the transom.
The motor’s propeller rotates at the bottom of a long shaft that can act on the transom as a lever, namely apply torsional forces on it (Torque), both vertically and horizontally, depending on the relative position of the hull and the direction in which the propeller is pushing –

  • Horizontally, when making a sharp turn or bumping against a wave hitting the hull on its side, and
  • Vertically, when a climbing up a big wave or coming down on the other side.
The following examples may clarify this –

1. A boat that makes a sharp turn

When a boat makes a sharp right turn, the propeller rotates the transom clockwise, while the water resisting the right motion of the hull applies a torsional force on the transom in the opposite direction, namely counter-clockwise.

2. A boat’s bow “dropping” on the back side of a big wave, or wake

In this case there is a sudden change, and there is no water to resist the hull’s forward motion and push its bow upward, so the bow drops abruptly. The result is a sharp change in the hull’s angle, with the bow now pointing downward and the stern being higher than the bow. In such case, the dropping bow pulls the transom’s top forward, generating considerable stress.

In both these cases, the torsional forces and the resisting forces create stress in the boat’s transom. The stronger the motor, the faster the boat goes, and the bigger and more sudden the change, the more stress.
If the transom and motor mount are not properly designed and built, this could lead to a structural failure in either, and to serious trouble.

Danger to the transom during transportation

The boat’s transom is exposed to powerful torsional forces even when it is not in use, particularly during transportation, be it on a trailer or a pickup truck bed.
In these conditions, the heavy motor hangs outside the boat from its transom, and it can be exposed to severe shocks when the transporting vehicle goes over bumps in the road, even at low speed. The heavier the motor and the higher the transporting vehicle’s speed, the stronger the impact on the transom, and consequently, the potential damage to the boat.
For this reason, using a motor guard is very much recommended.

Overpowering a boat

A boat is considered Overpowered when it is powered by a motor, or motors that can generate more power than the HP for which the manufacturer rates this boat. In some states, overpowering a boat is illegal, since it is perceived as hazardous, not just in terms of potential structural failure of the transom, or motor mount, but also a failure of the driver to drive the boat safely in speeds that exceed the maximum speed attainable with a motor whose HP falls within the HP limit for which the boat is rated.

Any boat owner considering overpowering their boat should be aware of the legal and technical aspects of such action. Another factor to consider is the attitude of companies that insure boats, which may not be favorable.

Physical factors

The physical factors that require attention are the motor’s additional weight, and the additional power that it can generate beyond the weight and power (HP) of a motor for which the boat is rated. The bigger these differences, the bigger the risk, and the more comprehensive the required modifications in the transom and stern.
In many cases, simply making the transom thicker is not enough, and more structural work may be required to firmly attach the transom to other areas in the stern, such as its sides, its bottom, and other rigid structures that may support it. Spreading the loads that the transom is required to sustain may be as important as reinforcing it, and in some cases even more so.
In case of a small twin-hull boat such as the S4 micoskiff or W720 kayak-skiff, these areas in the stern may include the rear hull tips, and the rear end of the saddle seat structure.

Note that Wavewalk recently updated the article Motor Power Rating For Canoes, Kayaks, and Small Boats, about rating for maximum motor power, and overpowering boats. However that article does not elaborate on structural issues discussed here.

The Ultralight W720 Kayak-Skiff Walkaround Video

The W720 is a successful embodiment of the Wavewalk Micronautical design.  It is the perfect blend between kayak and boat performance – The ease of paddling with the stability and control required for motorizing.
As a kayak, the W720 tracks like no other, thanks to its long waterline, and it’s more stable than any other kayak out there, including fishing kayak behemoths that are over 40 inches wide, and weigh north of 120 lbs.
As a motorboat, it is nimble and responsive, and the easiest to launch and beach.
On top of this, the W720 is comfortable to ride in, thanks to its high saddle seat that stretches along the center line of its cockpit, and allows for up to three passengers to enjoy paddling and/or motorizing on lakes, rivers, flats and estuaries.

It definitely stands apart from the crowd of electric motor kayaks that neither look nor perform well.

The W720 Kayak Skiff does not offer the amazing seaworthiness of the S4 Microskiff, not its speed, but it is the best solo and tandem kayak out there, and it works perfectly as an ultralight motorboat, including as a dinghy (boat tender).

This video shows the W720 outfitted with a 3 gallon fuel tank, This quantity is enough for a whole day of nonstop driving with an outboard motor up to 3 HP (practically 2.5 HP). This tank was too large to fit inside one of the W720 hulls, but it conveniently fits  on top of the saddle, between the motor and the driver.

Hands-on: DIY articles

Most people don’t design their own microskiff, or portable boat, but many love to outfit it, and build accessories for it  –

Wavewalk recently published several articles on subjects related to Do It Yourself (DIY) projects.

Kurt Raimer’s from WA state outfitted his S4 microskiff for fishing salmon and trout:

Bill Seay’s report on his Easy-Load for his S4:

Terry Wilkison’s new white S4 is loaded with DIY upgrades:

S4 microskiff top view

Read full article: https://wavewalk.com/blog/2023/04/19/first-tandem-spearfishing-trip-in-my-white-s4-microskiff-guam/

How To Trim My S4 Microskiff For Speed? »

Design, including boat design, is about choosing the formula that would best answer the requirements of the user. In the case of the S4 Microskiff, the requirements for extreme portability (easy car topping) and versatility (ability to paddle) led to a limitation on engine power, mostly due to its weight.
Many boat owners care about being able to reach the maximum speed that their boat allows for, so they spend much time and money on trimming their boat, and this article offers S4 Microskiff owners information and hand-on advice on how to trim their boat for speed.

One of the fist questions that future S4 owners ask is about motorizing, and more specifically, what size motor would best fit their needs –

3.5 HP or 6 HP outboard motor for my S4 microskiff? »
Wavewalk and its dealers sell the S4 microskiff non-motorized, and it’s up to the boat owner to choose what type and size of motor they will use. Wavewalk offers plenty of info and good advice on these subjects, and this is the latest article in this series.

It’s worth mentioning here that Wavewalk’s website offers dozens of articles including on subjects related to boat design and DIY projects.
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